Say nothing at all. If he presses you for an answer, say 'I am considering what you've written,' and use all your avoid-R-talks-like-the-plague techniques to get yourself out of any further conversation. You can consider it for as long as you like.
A marriage, or working on a marriage, with your H is not on the cards right now. Your choices are between the current state of affairs, a more formalised separation, or you pulling the trigger on D. And you can decide which of those work for you on your own schedule, as best meets your needs, without having to have any more talk with your H.
